The invention relates to a voltage supply for a sensor unit, particularly an acceleration sensor unit in an occupant protection system in a motor vehicle, with a voltage supply unit which operates at an internal switching frequency having a capacitor connected in parallel with the output, with a sensor unit which operates at an internal switching frequency and having at least one capacitor connected in parallel with the power supply input, with a line connecting the output of the voltage supply unit to the power supply input of the sensor unit and with a filter having a filter component contained between the branch points of the capacitors in the line.
FIG. 6 shows a known voltage supply. A voltage supply unit 10 operating at an internal switching frequency is connected to a sensor unit 14 via a line 12. The sensor unit 14 contains, by way of example, an acceleration sensor, such as is used in occupant protection systems in motor vehicles. Such acceleration sensors usually operate with a switched capacitor filter (not shown) and are sensitive to voltage fluctuations, particularly if their switching frequency matches the switching frequency of the voltage supply unit or matches a multiple thereof. In principle, there is always a risk when the switching frequency of the sensor unit or an integer multiple thereof matches the switching frequency of the voltage supply unit or matches an integer multiple thereof.
Examples of causes of fluctuations in the supply voltage produced at the output of the supply voltage unit 10 are:
1) cyclic voltage fluctuations in time with the switching frequency;
2) random noise;
3) short voltage spikes.
The cyclic interference, in particular, is critical for the sensor. Random noise is not tied to any particular frequency and the switching spikes can be blocked out by a simple ceramic capacitor directly at the input of the sensor unit.
If the sensor unit reacts to voltage fluctuations, this can result in erroneous signals at the sensor output which, in turn, can cause the following problems:
a) a routinely executed sensor test reports errors;
b) a crash algorithm is triggered;
c) the acceleration signal measured by the sensor is corrupted by interference.
To eliminate problem type c), the voltage transmitted over the line 12 is filtered such that the corruptions in the sensor output signal which are caused by voltage fluctuations do not exceed an acceptable degree. Such filtering likewise eliminates problems a) and b).
For the purpose of filtering, besides possible other components, a smoothing capacitor 18 of, typically, 10 to 100 xcexcF is connected in parallel with the output 16 of the voltage supply unit 10 (FIG. 6), whose output voltage is typically 5 V. Upstream of the smoothing capacitor""s branch point, there is typically an inductor (not shown). Connected in parallel with the power supply input 20 of the sensor unit 14 is a capacitor 22 which is present for reasons of electromagnetic compatibility and additionally further reduces switching spikes not suppressed by the smoothing capacitor 18 in the frequency range from approximately 100 kHz to a few MHz. The capacitor 22 typically has a capacitance of from 10 to 100 nF. In addition, there is an LC filter comprising a coil 24 and a capacitor 26, in particular for the purpose of suppressing the cyclic voltage fluctuations which arise in time with the switching frequency of the voltage unit 10. The inductance of the coil is typically between 10 and 100 xcexcH; the capacitance of the capacitor is between 10 and 100 xcexcF, for example. The circuit shown in FIG. 6 is used to produce, for a switching frequency in the voltage supply unit 10 in the region of 100 kHz and switching frequencies in the acceleration sensor 14 in the region of 70 kHz, a voltage constancy at the input 20 of the sensor unit 14 which ensures the operational reliability of the sensor unit 14.
On account of their high capacitances, the capacitors 18 and 26 are typically tantalum capacitors. The capacitor 22 is typically a ceramic capacitor on account of the RF response.
One problem of the circuit shown in FIG. 6 is that it is comparatively complex on account of the LC filtering.
It is accordingly an object of the invention to provide a voltage supply for a sensor unit and an acceleration sensor unit with such a voltage supply, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which can be produced economically and with little complexity while having a high level of operational reliability.
With the forgoing and other objects in view there is provided, in accordance with the invention, a voltage supply for a sensor unit, comprising a voltage supply unit operating at an internal switching frequency and having an output, a first capacitor connected in parallel with the output forming a branch point, a sensor unit operating at an internal switching frequency and having a power supply input and at least one second capacitor connected in parallel with the power supply input forming another branch point, a line connecting the output of the voltage supply unit to the power supply input of the sensor unit, a filter having a filter component contained between the branch points of the capacitors in the line and the filter component being formed by a resistor connected together with the second capacitor being connected in parallel with the power supply input of the sensor to form the filter.
It has been found, surprisingly, that the complex LC filter described with reference to FIG. 6 can be replaced with a simple nonreactive resistor without there being any risk of impermissibly large voltage fluctuations arising at the input of the sensor unit 14 which could threaten fault-free operation of the sensor unit 14. If required, the capacitance of the filter capacitor needs to be increased in relation to that of the capacitor used in the prior art, which can also be done by connecting a plurality of capacitors in parallel, e.g. 3 times 470 nF.
In accordance with another feature of the invention, a sensor unit is one of a plurality of sensor units connected in parallel on the line. This relates to an advantageous development of the voltage supply according to the invention, with the filter being able to have just one resistor in accordance with the resistor forming part of the filter is common to all of the sensor units.
In accordance with a further feature of the invention, a second capacitor connected in parallel with said power supply input of said sensor unit is a ceramic capacitor.
In accordance with an additional feature, the filtered voltage may advantageously be used as reference voltage for a microcontroller which is used for converting the output signals from the sensor unit(s).
With the objects of the invention in view, there is also provided an acceleration sensor unit for an occupant protection system in a motor vehicle including a voltage supply, comprising a voltage supply unit operating at an internal switching frequency and having an output, a first capacitor connected in parallel with the output forming a branch point, a sensor unit operating at an internal switching frequency and having a power supply input and at least one second capacitor connected in parallel with the power supply input forming another branch point, a line connecting the output of the voltage supply unit to the power supply input of the sensor unit, a filter having a filter component contained between the branch points of the capacitors in the line and the filter component being formed by a resistor connected together with the second capacitor being connected in parallel with the power supply input of the sensor to form the filter.
The invention may be used to advantage anywhere where natural frequencies of a voltage supply unit, which cause voltage fluctuations, are adversely superimposed on natural frequencies of a sensor unit (harmonics).
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a voltage supply for a sensor unit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.